This invention relates to integrated circuit capacitors, and more particularly to voltage-variable integrated capacitors.
Variable capacitors or varactors have been used for many years in tuning circuits. For example, radio receivers once used variable capacitors that adjusted a band-pass frequency in a tuning circuit, allowing one radio station to be amplified while suppressing other radio frequency signals. The amount of capacitance was varied by a user turning a dial that rotated half of the metal plates of the capacitor. As the user rotated the plates, the overlap area between adjacent metal plates varied, thus varying the capacitance value. FIG. 1 shows a symbol for a variable capacitor or varactor.
More recently, capacitors have been integrated onto larger integrated circuit (IC) chips. Often a capacitor is formed from a metal-oxide-semiconductor (MOS) field-effect transistor. FIG. 2 shows a prior-art MOS capacitor. The source and drain terminals of the MOS transistor are shorted together and form one terminal ND of the capacitor, with the MOS transistor""s gate forms the second capacitor terminal NG. The very thin gate oxide provides a large capacitance value in a small area.
The capacitance can be varied by changing the size of the MOS transistor. The circuit designer can select the desired capacitance by selecting the W and L values of the capacitor-transistor. However, once the circuit is manufactured, the capacitance is fixed.
A variable capacitor can be made by including several capacitors in parallel on the IC chip. An array of switches or pass transistors can selectively couple the parallel capacitors to a circuit node. Electronic signals can be applied to the gates of some of the pass transistors to electronically connect the selected capacitors to the circuit node. Other electronic signals can turn off other pass transistors, disconnecting their capacitors from the circuit node. As more of the parallel capacitors are connected to the circuit node, the capacitance increases. Thus the total capacitance attached to the circuit node can be electronically selected. The electronic signals can be coded binary signals such as a thermometer code.
While such a binary-controlled variable capacitor is useful, an analog-voltage controlled variable capacitor is desired. A variable capacitor that changes its capacitance value based on an applied voltage can allow for a wider range of capacitances, rather than a limited quanta of capacitances selected by binary control signals. What is desired is a voltage-variable capacitor that can be integrated with circuits using common MOS processes.